With RFID transponders, such as RFID tags a rectifier is used to extract electrical energy out of an incoming RF signal. The rectifier is used to convert the RF energy into a DC voltage that is configured to power a digital unit or logic unit or to power a processor of the RFID circuit. Common architectures for rectifiers use a Dickson-style charge pump and cascade multiple stages together. Each stage of such a charge pump can be comprised of two diodes in combination with two capacitors.
In current designs, it is usual to implement MOSFET transistors to improve efficiency and to enable integration of the rectifier with CMOS architecture. To accommodate a large dynamic range of RF input energy it is typical to follow the rectifier with a limiter that is configured to limit the output voltage to a level that prevents damage to the processor or logic unit of the RFID circuit when a high power RF signal is present at an antenna of the RFID circuit. However, such a limiter can be rather inefficient. It may waste power and/or it may induce a large amount of fluctuations or ripples on an output electrically connected to the logic unit or processor.
One deficiency of a limiter-based voltage regulation for an RFID circuit is that during an amplitude modulation (AM modulation) the incoming RF signal is switched on and off to encode data for the RFID circuit. In such situations the limiter may waste energy by shunting extra energy to ground during the transitions from the RF on state to the RF off state and vice versa.
Additionally, the rectifier may exhibit a reverse current leakage that may also siphon energy away from the recovered DC voltage during the time period when no RF incoming signal is present. Also due to the switching action of the RF input signal and the limited bandwidth of the limiting circuitry comparatively large fluctuations or ripples may be induced onto the DC output signal. Such fluctuations or ripples may cause problems with sensitive analogue circuitry of the RFID circuit or RFID tag. Such fluctuations or ripples on the DC output of the regulator or limiter become even worse when the incoming RF field is comparatively strong. This is because the limiter must transition very fast from shunting a lot of excess current to ground during the time when the RF field is present to not shunting any current while the RF signal is switched off.
Document U.S. Pat. No. 7,538,673 B2 describes a voltage regulation circuit based on low drop oscillators (LDO). Such a solution is limited in bandwidth and with regard to so called headroom losses. The implementation of numerous LDOs involves four active circuits that may add complexity, die array and increased current consumption.
It is therefore desirable to provide an improved voltage regulation circuit for an RFID circuit that exhibits a reduced degree of complexity and which comprises only a limited number of active components. It is further desirable to provide a faster reaction time and a wider bandwidth. Also, less die array should be used by the voltage regulation circuit. The voltage regulation circuit should also exhibit minimal headroom losses and lower current consumption.